1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to a light-guiding plate, backlight module and liquid crystal display device.
2. The Related Arts
Due to the advantages of reduced thickness, light weight, low power consumption and low radiation, the liquid crystal display (LCD) devices find wide applications in the field of displaying for computers, mobile phones, electronic dictionary, and liquid crystal televisions. With the recent light efficiency improvement in liquid crystal display devices, a new trend of the liquid crystal display device is to reduce the number of light sources so as to reduce the structure of the backlight module used in the liquid crystal display device. As shown in FIG. 1, a backlight module of liquid crystal display device usually comprises a light-guiding plate 11 and light sources 12. The light sources 12 are disposed at the sides of the light-guiding plate 11. The backlight module progresses from the original four-edge lit ((a) of FIG. 1) to the two-long-side lit ((b) of FIG. 1), two-short-edge lit ((c) of FIG. 1) and single-long-edge lit ((d) of FIG. 1), or even to the single-short-edge lit in mass production ((e) of FIG. 1).
As the luminance of the light source improves, the further development based on the short-edge lit will be towards the corner lit, that is, light sources disposed at corners of the light-guiding plate. FIG. 2 shows a plurality of possibilities for corner lit at the light-guiding plate. In FIG. 2, (a) is to dispose a light source 22 at a corner of the light-guiding plate 21; (b) is to dispose a light source 22 respectively at two diagonal corners of the light-guiding plate 21; (c) is to disposed a light source 22 respectively at two corners at the same side of the light-guiding plate 21; and (d) is to dispose a light source 22 respectively at four corners of the light-guiding plate 21. In the corner-lit design, an important issue is how to import the light from the corners of the light-guiding plate to emit uniformly from the light-emitting surface of the light-guiding plate. For small-sized liquid crystal display device, because of the short propagation distance of the light, the mesh dots can be disposed on the bottom (opposite to the light-emitting surface) of the light-guiding plate. For example, the uniform light-emission from the light-emitting surface of the light-guiding plate can be realized by change the size or the density of the mesh dots. However, for large-sized liquid crystal display device, because of the longer propagation distance of the light, the mesh dots disposed on the bottom (opposite to the light-emitting surface) of the light-guiding plate cannot realize the uniform light-emission from the light-emitting surface of the light-guiding plate.
In addition, the light source in actual application, such as, light-emitting diode (LED) usually has specific scattering angle. When emitting light at the corners of the light-guiding plate, the scattering angle of the light from the LED after entering the light-guiding plate is smaller than the scattering angle of the original light source. As shown in FIG. 3, the LED 32 as the light source usually has a scattering angle of 120°. The deflection ratio of a light-guiding plate 31 is usually larger than 1. For example, when the light-guiding plate 31 is made of PMMA, the deflection ratio is about 1.49. When the emitted light enters the light-guiding plate 31, the scattering angle in the light-guiding plate 31 is smaller because the light will be deflected (shown as arrow in the figure). As a result, the corner A and corner B neighboring the light-emitting corner on the light-guiding plate 31 cannot receive light and becomes dark, which severely affects the uniformity of the light-emission of the light-guiding plate 31.